git Gui: Add directly calling merge tool from gitconfig

* git Gui can open a merge tool when conflicts are
    detected. The merge tools that are allowed to
    call have to be hard coded into git Gui
    althgough there are configuration options for
    merge tools git in the git config. Git calls
    the configured merge tools directly from the
    config while git Gui doesn't.
* git Gui can now call the tool configured in the
    gitconfig directly.
* Can be enabled through setting
    gui.mergeToolFromConfig
* Disabled by default, since option is most likely
    never set
* bc3 and vscode tested

Documentation/config/gui.txt

@@ -55,3 +55,7 @@ gui.blamehistoryctx::
 	linkgit:gitk[1] for the selected commit, when the `Show History
 	Context` menu item is invoked from 'git gui blame'. If this
 	variable is set to zero, the whole history is shown.
+
+gui.mergeToolFromConfig::
+	If true, allow to call the merge tool configured in gitconfig
+	in git gui directly.

git-gui/lib/mergetool.tcl

@@ -272,8 +272,15 @@ proc merge_resolve_tool2 {} {
 		}
 	}
 	default {
-		error_popup [mc "Unsupported merge tool '%s'" $tool]
-		return
+		if {[is_config_true gui.mergetoolfromconfig]} {
+			set path [get_config mergetool.$tool.path]
+			set cmdline_config [get_config mergetool.$tool.cmd]
+			set cmdline_substituted [subst -nobackslashes -nocommands $cmdline_config]
+			set cmdline [lreplace $cmdline_substituted 0 0 $path]
+		} else {
+			error_popup [mc "Unsupported merge tool '%s'" $tool]
+			return
+		}
 	}
 	}